Newly developed highly porous gas storage materials suitable for cryogenic adsorption and desorption of gases are known in the Art. Such materials are, for example, activated charcoal, metal organic frameworks (MOFs and MILs), nano-cubes, coordination polymers (CPs), prussian blue analogues, or polymers of intrinsic microporosity. A description of highly porous gas storage materials can be found in the articles written by Professor Yaghi of the University of Michigan, published in Science magazine. (Systematic Design of Pore Size and Functionality of Isoreticular MOFs and Their Application in Methane Storage, Science Vol. 295, 18 January 2002; Hydrogen Storage in Microporous Metal-Organic Frameworks, Science Vol. 300, 16 May 2003). Also, in a press release by Dr. Ulrich Muller, of BASF, 28/29 10, 2002, “Nano-cubes for Hydrogen Storage” MOFs are described here as “Nano-cubes”. Highly porous polymers suitable as gas storage materials are also described in an article in Materials Today, April 2004, “Microporous Polymeric Materials”. All these highly porous gas storage materials have surface area densities from 3,000 m2/g (activated charcoal, MOF5) to more than 4,500 m2/g (MOF177, NATURE, Vol. 427, 5 February 2004, “A Route to High Surface Area Porosity and Inclusion of Large Molecules in Crystals”). Recently developed MOFs (MILs), such as nano-cubes, have shown surface area densities greater than 5,000 m2/g, ie., MEL 101 with 5,600 m2/g (MIL-101 is a new, unusually porous material whose unit cell has an unprecedented volume of about 702,000 cubic Angstroms, meaning that the solid is about 90% empty space once the solvent molecules normally filling its pores are removed. It also boasts pores that are 29 or 34 Angstroms across and an internal surface area of 5,900 m2/g (Science 2005, 309, 2040).
Due to their high porosity (typical mass densities ranging from 0.3 to 0.6 g/cm3) and high surface area, highly porous gas storage materials could be used for the storage of gases, such as methane and hydrogen. The gas is adsorbed (using very weak van der Waals forces) on the large surface areas as a monolayer (for moist cases). These highly porous gas storage materials are usually fine powders. To increase the volumetric density, they could be compressed to be formed into fine or course granulated material (pellets). This granulated material has a higher mass density, eg., about 0.7 g/cm3, but also an up to 30% reduction in the surface area. These highly porous gas storage materials may be filled into a pressure vessel. The heat generated during the adsorption process (adsorption energy between about 3 and 6 kJ/mol H2 with MOFs and about 6 kJ/mol H2 with activated charcoal) should be compensated by a heat exchanger. There may be ambient temperature and cryogenic operation modes depending on the gas, for example H2 or natural gas.
Cryogenic gas storage containers have become especially interesting to the automotive industry through the development of these aforementioned highly porous gas storage materials. The cryogenic storage of gaseous energy carriers, such as natural gas (methane) and hydrogen is especially interesting for automotive applications utilizing, for example, fuel cells or internal combustion engines since a high degree of development potential is available regarding tank volumes (required space), weight, and safety in conjunction with these aforementioned highly porous gas storage materials.
The stored gas is removed from the cryogenic gas storage containers by desorption. Desorption occurs by a suitable supply of heat energy and by a reduction, usually, of the gas pressure.
Previously, cryogenic gas storage containers were only built for purposes of research or material development whereby desorption of the stored gas is realized through the use of direct, internal electric heaters with heating wires imbedded in the gas storage media or heat exchangers with embedded heat exchanger tubes in the gas storage media.
An energy saving desorption strategy is not possible or possible only within limits through the introduction of heat for desorption using an electric heater since electrical energy for the electric heater must be supplied, with a loss of efficiency. Also, a space saving desorption strategy is possible only within limits utilizing embedded heat exchangers since a large number of heat exchanger tubes must be placed in the cryogenic gas storage container in order to transfer the necessary quantity of heat. This unacceptably increases the volume of the cryogenic gas storage container and only an incomplete uniform temperature distribution is achieved, with high costs.
Furthermore, an introduction of the entire heat requirement for desorption by an electric heater or by heat exchanger tubes is hampered in that direct heat contact with the highly porous gas storage material is inhibited through marginal heat contact of the electric heater or heat exchanger tubes with the surrounding gas storage media. Thus, a high temperature profile is necessary for the required desorption heat flux whereby significantly higher heat energy must be introduced into the cryogenic gas storage container than would be necessary for the pure desorption of the gases.
Cryogenic gas storage containers are also developed as testing devices, in which the gas storage material is enveloped by a sheathing made of liquid nitrogen. With desorption, the corresponding heat quantity is removed from the liquid nitrogen, to prevent too low a cooling of the gas storage material thereby maintaining the gas stream during desorption by pressure relief. Thus, today there is no existing optimum heating or space saving strategy for desorption of stored gas from cryogenic gas storage containers.
Even, for example, for automotive applications utilizing, for example, fuel cells, an optimal energy and space saving strategy for desorption of stored gas from cryogenic gas storage containers is not known, whereby the ambient heat and/or heat dissipation of an internal combustion engine and/or a fuel cell is utilized. The heat dissipation of an internal combustion engine or a fuel cell cannot be directly introduced in heat exchanger tubes within the cryogenic gas storage container since, for example, the heat transfer medium, coolant or water, would freeze. Even ambient air cannot be introduced directly into the heat exchanger tubes since, for example for cryogenic storage at 80 K, a separation and liquefaction of the nitrogen and oxygen gases would occur.
Accordingly, what is needed in the art is an optimal energy, weight, and space saving strategy for desorption of stored gas from cryogenic gas storage containers whereby, for example for automotive applications utilizing fuel cells, the ambient heat and/or heat dissipation of an internal combustion engine and/or a fuel cell is utilized.